High School Chemistry : Help with Enthalpy

Example Questions

Example Question #3 : Thermochemistry And Energetics

Consider the following balanced reaction:

What is the change in enthalpy if  of solid carbon is used in the above reaction?

Explanation:

The enthalpy of  describes the amount of heat when the amount of carbon in the balanced reaction (two moles) is used. Since only  of carbon are used, we can find how much heat is released.

When two moles of carbon are used,  are released. Two moles of carbon is equal to  of carbon, based on carbon's atomic mass.

Knowing this, we can set up proportions in order to determine how much heat is released by  of carbon.

So  of carbon results in  of heat being released to the surroundings.

Example Question #1 : Help With Enthalpy

Consider these two half reactions:

Step 1.

Step 2.

Based on these half reactions, find the enthalpy for the following reaction:

Explanation:

Hess's law states that the enthalpy of the total reaction is equal to the enthalpy of the steps required to get to the total reaction, regardless of the path that is chosen. This means that we can combine the two half steps with known enthalpies in order to solve for the enthalpy of the main reaction.

Step 1.

Step 2.

Total:

Since step 1 results in two moles of liquid water, we need to use the second step twice in order to replace them with two moles of water vapor.

Combined:

Since the total reaction is created by step 1 occurring once and step 2 occurring twice, we can write the enthalpy as:

Use the given enthalpies of the steps to calculate the total change in enthalpy.

Example Question #1 : Help With Enthalpy

How much energy is required to heat of from to ?